1. Field of the Invention
The present invention relates to an apparatus for rewriting a memory in a vehicle mounted ECU (Electronic Control Unit) through communications in order to allow a control program for the vehicle mounted ECU to be changed with ease.
2. Description of the Related Art
Recently, a lot of ECUs have come to be installed in vehicles such as motor vehicles in the background of the advancement of electronic technologies, especially the microcomputer application technology. For instance, such ECUs include engine control ECUs, transmission control ECUs (hereinafter called A/T ECUs), navigation ECUs, etc.
In the microcomputers of these ECUs, “control data” and “control programs” that seem to be optimal at the stage of development are stored. For instance, the following data are stored in the engine control ECUs.                Drive timing control data for continuously variable valve timing device (WT) (for improvement in engine power).        Air/fuel ratio control data (for further cleaning exhaust gases).        Ignition timing data (for improvement in engine power).        Control programs for controlling the engine and transmission by using the above data.        
In addition, in the navigation ECUs there are stored navigation data and programs such as “map data”, “route searching programs”, and the like.
However, more optimal data and more suitable control programs than those at the stage of the initial development might be obtained by performing the review of data, control logic, etc., for these control ECUs even after they have been produced after the completion of their developments. In this case, it is extremely difficult to change the programs in the ECUs that came onto the market after their manufacture. That is, it is very time-consuming work that the vehicles having these ECUs installed thereon are recalled and subjected to the rewriting of their memories one by one after these products have been manufactured. Therefore, fresh or new data cannot be used for the vehicles of the initial type or model which have already been manufactured, and the new data are installed only on the vehicles which have been subjected to minor changes and later on. As a result, there will arise a problem that the vehicles with control data of the old models installed thereon, which are of quite the same in hardware as those of new models, can not enjoy the advantages of the latest data in respect of fuel cost, emission control measures, environmental preservation, etc.
In view of the above problem, it is thought that programs are made to be upgraded through communications. For such a technique, reference is made to Japanese patent application laid-open No. Hei 11-143720 as one example of an invention related to the upgrade of audio-related ECUs through communications. This prior art document describes a method in which communications are permitted by pressing a hidden command button for the upgrade of an ECU, so that upgrading is executed through radio communications. However, this prior art method involves another problem that an operator has to take the trouble of pressing the hidden command button of the vehicle to be rewritten and that no measure is taken for the case where there take place poor communications such as malfunctions, etc., due to noise during communications. Moreover, no means is mentioned for distinguishing the environments where trouble might occur while communicating, so it is impossible to prevent communication failures.
In addition, when the data stored in the ECU is rewritten, an electric malfunction, the destruction of communication signals or the like might be caused due to reduction in the battery voltage or by driver's operation of the key switch which might induce such battery voltage reduction. Further, the vehicle would be subjected to various external disturbances while travelling so that normal data communications might be obstructed occasionally.